Add chat pins and Later saved items

[rogerchappel]

Summary

• add DB-backed chat message pins and generic saved items for Later
• add authenticated APIs for pinning, saving, updating, and deleting saved items
• add chat message actions, pinned-message strip, and a Later workspace page/nav entry

Verification

• pnpm typecheck
• pnpm lint (passes with existing warnings)
• pnpm build (passes with existing Turbopack NFT warning in next.config.ts import trace)
• git diff --check origin/main..HEAD